Forces,moments and stability

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Forces in equilibrium

Vectors and Scalars                                        pg90

Finding the resultant force of two forces acting on a object or each other in a given direction (angle)

Use pythag or trig to work out . 

A vector is any physical quantity that has a direction as well as a magnitude.

Scalar is any physical quantity that is not directional 

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Balanced forces

Equilibrium of a point object                                               pg94

When forces act on a point object, the object is in equilibrium (at rest of moving at a constant velocity) e.i balanced

support force S = Weight W 

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Moments

All moments have a turning effect and a centre of mass                             pg97

turning effect is the force at a pivot

centre of mass is the point at where an object is balanced (mid point of weight)

The moment of a force about any point is defined as the force x the perpendicular distance from the line of action of the force to the point 

principle of moents: sum of clockwise = sum of anticlockwise 

e.g for a sesaw to be balanced both side must have the same forces acting on them about the pivot point. 

moment of a force = f x d

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Stability

If a body in stable equilibrium is displaced the released,it returns to its equilibrium position. 

For example if a coat hanger from a support is displaced slightly, its swings bk to its equilibrium position

the centre of mass is always the equilibrium position of a object plus any force acting

tilting :

is where a force acts on a object which forces it to raise on one side 

force applied, centre of mass and height of object

Toppling:

finding the toppling point where the object has tilted so much it topples due to the force and centre of mass on a pivot point.

Slopes also act on this.                                                           pg100

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Equilibrium rules

For an object to be in Equilibrium all the forces acting on it must end up balanced           

weight W (centre of mass), support force S and friction F are the forces acting. 

f1 + f2 +f3 = 0 e.i it must balance out . 

normally calculated on slopes using free body diagrams which is a object with a centre of mass causing friction and a  support force perpendicular to the friction.               pg104

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static calculations

Pg 108 questions 

Static calculations finding the resultant forces on a object from a number of forces acting on it. 

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